1. Field of the Invention
The present invention relates to alignment metrology, and in particular to apparatus and methods for measuring alignment and/or overlay accuracy of images formed in or on a substrate.
2. Background Information
In semiconductor manufacturing, the processing steps for fabricating a semiconductor device (e.g., an integrated circuit) include exposing a substrate, such as a semiconductor wafer coated with photosensitive material, using a lithographic exposure apparatus. This exposure involves forming images at precise locations on the substrate. In one example, the substrate, which resides on a substrate stage, is aligned to a reticle residing on a reticle stage. The reticle includes a pattern of a particular device layer, as well as alignment marks. The alignment is performed using an alignment apparatus, such as that disclosed in U.S. Pat. No. 5,621,813, which patent is incorporated herein by reference.
After aligning the substrate, the reticle is illuminated with radiation and the reticle pattern is imaged in the photoresist. The exposed photoresist is then developed and the substrate etched to transfer the resist pattern to the substrate.
The region of the substrate over which the reticle is imaged is sometimes referred to an “exposure field”. This alignment and exposure process can be performed on a variety of lithography apparatus, such as step and repeat, step-and-scan, contact and proximity apparatus.
Typically, the first device layer is aligned to a feature or mark on the substrate, such as a flat or notch and the substrate edge. The subsequent layers are then aligned relative to this first layer and/or to each other. “Alignment”, as the term is used herein, refers to the position of the center of the exposure field relative to a reference location. “Overlay”, as the term is used herein, refers to the position and orientation of the exposure field relative to an ideal exposure field, and involves measuring the position of multiple points per exposure field. Thus for example, a particular exposure field can have perfect alignment but poor overlay, e.g., if the exposure field is imaged with rotation or distortion. “Alignment mark”, as the term is used herein, refers to a feature on either side of the substrate whose position can be measured to establish the degree of either alignment or overlay of a given exposure field.
To gauge the degree to which exposure fields are aligned and overlayed on the substrate, it is necessary to have the capability of making alignment and overlay measurements. This is typically accomplished by measuring the position of an alignment mark in the exposure field relative to another alignment mark or feature formed somewhere else on the substrate. By measuring alignment or overlay for a given substrate or set of substrates, adjustments can be made to the exposure tool to reduce or eliminate tool-related sources of alignment and/or overlay errors. Reducing such errors increases the likelihood that the device being manufactured by the exposure tool will perform to its specification.
Certain lithographic exposure apparatus perform exposures of the frontside of the substrate while aligning to features (e.g., alignment marks) on the backside of the substrate. Conventional apparatus and techniques for quantifying frontside to backside alignment performance involve measuring two marks at two widely separated points on the frontside of the substrate in relation to two corresponding backside marks. This typically involves viewing the alignment marks on the backside through the frontside of the substrate using, e.g., a single optical system operating with infrared light. However, this measurement technique cannot accurately measure mask (reticle) run-out or stepping-grid errors. Thus, the actual alignment performance over the entire substrate is not always properly characterized by conventional apparatus and techniques. Further, not all substrates are transparent enough to allow for both sides to be viewed by a single optical system positioned on one side of the substrate.
What is needed therefore are alignment apparatus and method for providing a more accurate characterization of alignment and overlay performance of an exposure tool.